The present invention relates to the electrical starter and medical diagnostic arts. It finds particular application for rapidly starting rotation of an x-ray tube anode. Rotating anode x-ray tubes are commonly found in conventional x-ray machines and computer tomographic scanners. It is to be appreciated, however, that the invention may also find application in digital x-ray scanners, other x-ray medical and non-medical devices, other motor and rotation speed control applications, and the like.
X-rays are produced when, in a vacuum, electrons are released, accelerated, and then abruptly retarded such as by impacting a target. In a conventional x-ray tube, a heating current, commonly on the order of 2-5 amps, is applied through a thermionic filament to create a surrounding electron cloud (i.e. released electrons). A high potential, e.g. 50-150 kilovolts, is applied between the filament and an anode to accelerate the electrons from the cloud to an anode target area. This acceleration of electrons causes a tube or anode current which is commonly on the order of 5-200 milliamps. To avoid thermal energy released as the electron hit the target area from overheating the anode, the anode rotates at a high speed during x-ray generation. When no x-rays are being generated, the anode may be allowed to decelerate. In a conventional x-ray device, after each x-ray photograph is taken, the anode is decelerated or braked to a stop and is restarted for the next photograph.
In a computed tomography scanner, the x-ray tube is rotatably mounted radially outward of an imaging region and opposite to a plurality of sensors. Commonly, a fan beam of x-rays is generated by the tube, passes through a subject, and impinges on the sensor array. Each time the x-ray tube is rotated about the subject, the sensors generate a series of views or scan from which an image is reconstructed. Before the x-ray beam can generate x-rays to start a scan, the anode must be brought up to its operating speed. After each set of image data is collected, the x-ray tube is turned off to stop generation of x-rays. If another scan is to follow quickly, the anode may be kept rotating at full speed between scans. Historically, however, after most scans, the anode was permitted to slow or stop. Thus, the x-ray tube anode goes through frequent accelerate and decelerate cycles.
The rotating anode of an x-ray tube has one or more natural resonant frequencies, i.e. angular velocities at which the anode vibrates excessively. The vibration may damage the anode, as well as shorten bearing life. It is accordingly, desirable to minimize the time spent at the resonant angular velocity during the start up time of the x-ray tube.
A single phase, one capacitor starter, employs a 180 Hz single phase invertor, which is utilized to bring the anode up to approximately 9000 RPM. This starter consists of a single 180 Hz invertor with a single output voltage. The starter output voltage is connected to a stator main winding directly and to an auxiliary winding through a series phase shifting capacitor. The capacitor causes the current in the auxiliary winding to lead the current in the main winding thereby producing output torque. However, the lead current is much less than an optimum phase shift of 90.degree..
A second type of rotor control, a bi-phased invertor, consists of two independent inverters running at 90.degree. phase shift. The output of one invertor is connected directly to the main winding and the other output is connected directly to the auxiliary winding. The bi-phased inverter drive circuit described in U.S. Pat. No. 4,829,551 of Resnick et al., provides for selective variation of the drive frequencies, i.e. a rotor speed controller.
Because a wide range of x-ray tubes with widely varying motor characteristics may be used with the starter, phase shift and ampere turn matching of the anode motor windings is often far from optimal. Multiple capacitors are commonly switched in and out to improve the matching for different stators and operating frequencies. As the current level in the windings change, the windings change temperature. The phase and relative magnitude of the current through the windings change correspondingly. This increases excessively the current necessary to accelerate the rotor to its operating speed in the short time allotted and to maintain the correct operating speed.
Both of the above mentioned drive circuits contain various undesirable characteristics during acceleration of the anode of an x-ray tube. The single phase, one capacitor starter demands a high volt-amp input from the main supply due to its poor efficiency of starting. The auxiliary winding conventionally includes fewer turns than the main winding and is wound of smaller wires. As a result, the auxiliary winding current is inherently more nearly in-phase with the line voltage than that of the main winding causing the phase difference to be significantly less than the desired 90 reducing starting torque. The small phase difference between the two winding currents makes the starting line current, which is the sum of the two winding currents, quite high. Additionally, since the single phase invertor has no way of determining the rotor speed, normal operation is to apply full power for a time period which is longer than necessary to bring the anode up to speed. This excess start time increases the heat input to the motor windings.
The bi-phase invertor motor control which is a no capacitor starter requires a long accelerating time due to an inadequate power factor. Normal operation for the bi-phase invertor also includes applying power for a period of time which is more than adequate to bring the anode up to speed, again causing excessive heat in the motor windings. Additionally, the longer acceleration time causes the anode to reside longer at mechanical resonance frequencies thereby causing faster degradation of the anode.
The present invention contemplates a new and improved method and apparatus for high-speed starting and stopping of an anode rotor in an x-ray tube, which overcomes the above referenced problems and others.